1 . Field of the Invention
The present invention relates to a semiconductor device and manufacturing method therefor, and more particularly to a semiconductor device in which a compound semiconductor containing In (indium) and a compound semiconductor not containing In form a heterointerface therebetween, and a manufacturing method therefor.
2. Description of the Related Art
In recent years, as broadband communications and public telecommunication networks using optical fibers have become widely used, there has been an increasing need to transmit a large amount of information at low cost. To meet such a demand, it is necessary to increase the amount of information that can be transmitted per unit time, that is, to increase the information transmission rate. Actually, the transmission rate has been progressively increased from 600 Mbps to 2.5 Gbps, to 10 Gbps.
Such an increase in the transmission rate of optical communications devices has led to an expansion in the market for optical communications networks for use not only in trunk systems but in access systems (offices, homes), requiring that the optical transceivers employ high-speed, high-efficiency, yet low-cost light emitting/receiving semiconductor devices.
A semiconductor laser (a semiconductor optical device), for example, is formed by growing a compound semiconductor in crystal form on an InP substrate or a GaN substrate or a GaAs substrate.
Typical compound semiconductors include Group III-V compound semiconductors in which Group III and Group V elements are combined together. Compound semiconductors having different composition may be produced by causing different numbers of Group III and Group V atoms to bond together.
These compound semiconductors include, for example, InGaAsP, GaAsP, GaPN, GaNAs, InGaN, AlGaN, AlGaInP, AlGaAs, AlGaInAs,and InGaP, which are formed on a compound semiconductor substrate such as that described above by a vapor phase epitaxy.
It should be noted that forming a laser diode (LD) or a light emitting diode (LED) requires forming a heterointerface at which different compound semiconductors meet.
Examples of heterointerfaces include, for example, AlGaN/InGaN (which refers to the interface between an InGaN layer and an AlGaN layer formed on the InGaN layer), AlGaAs/InGaP, and AlGaAs/GaAs.
In the case of such a heterointerface formed between two compound semiconductors, it would be ideal if the composition of the device were abruptly transitioned at the heterointerface from a first compound semiconductor to a second compound semiconductor. In reality, however, an altered layer is undesirably formed between the first and second compound semiconductors, and therefore there are two composition transitions at the heterointerface: from the first compound semiconductor to the altered layer and from the altered layer to the second compound semiconductor. The term “altered layer”, as used herein, refers to a layer unintentionally formed when a second compound semiconductor is formed on a first compound semiconductor to form a heterojunction. The composition of this layer cannot be controlled.
In a semiconductor device including a heterojunction, generally, the thinner the altered layer, the better the characteristics of the device. It would be ideal if no altered layer was formed and hence there were only one composition transition at the heterointerface between the first and second compound semiconductors.
One example of a heterointerface used in an optical device is AlGaAs/GaAs. In this heterointerface (AlGaAs/GaAs), the first compound semiconductor is GaAs and the second compound semiconductor is AlGaAs. This means that the heterointerface portion includes only two Group III elements (namely, Al and Ga) and one Group V element (namely, As). When such a heterointerface portion is grown in crystal form, no altered layer is formed, or only a very thin altered layer is formed, since an abrupt transition can be made from the Group III element or elements of one compound semiconductor to those of the other compound semiconductor, eliminating the problem of degradation of the optical device due to formation of an altered layer.
However, such heterointerfaces as AlGaN/InGaN and AlGaAs/InGaP have the following problems.
(i) In these heterointerfaces, the lower layer contains In and Ga as Group III elements, while the upper layer contains Al and Ga as Group III elements. That is, there is a transition from a layer containing In (the lower layer) to a layer not containing In (the upper layer).
(ii) In the case of AlGaAs/InGaP, the lower layer contains P as a Group V element, while the upper layer contains As as a Group V element. That is, there is a transition from one Group V element (P) to another Group V element (As).
These problems (i) and (ii) prevent abrupt transition from the Group III and V elements of the lower layer to those of the upper layer at the heterointerface when the device is grown in crystal form. This is due to diffusion of In from a compound semiconductor containing In (the lower layer) to a compound semiconductor not containing In (the upper layer), segregation of In at the heterointerface, mutual diffusion of In and Group V atoms, and mutual diffusion of different Group V atoms promoted by In.
As a known example of a semiconductor device formed to prevent diffusion of In from the In-containing Group III-V compound semiconductor layer and a manufacturing method therefore, there is a disclosure in which an In diffusion blocking layer made up of a Te-containing Group II-IV compound semiconductor layer is formed on an In-containing Group III-V compound semiconductor layer, and a Group II-IV compound semiconductor layer is formed on the In diffusion blocking layer. (See, e.g., paragraph [0006] and FIG. 1 of Japanese Laid-Open Patent Publication No. 2000-91707.)
A semiconductor device, for example, a laser diode (LD), has a structure in which the following layers are laminated to one another over an n-type GaAs semiconductor substrate by a vapor phase epitaxy (n-type, p-type, and i-type (undoped) being hereinafter abbreviated as “n-”, “p-”, and “i-”, respectively): an n-AlGaAs n-buffer layer; an n-AlGaInP n-first cladding layer; an n-AlGaAs n-second cladding layer; an i-AlGaAs n-side guiding layer; an active layer structure including an AlGaAs quantum well and a barrier layer; an i-AlGaAs p-side guiding layer; a p-AlGaAs p-first cladding layer; a p-GaInP p-second cladding layer; a p-AlGaInP p-third cladding layer; a p-InGaP p-BDR (Band Discontinuity Reduction) layer; and a p-GaAs contact layer.
In this laminated structure of the LD, the junction between the n-AlGaInP n-first cladding layer and the n-AlGaAs n-second cladding layer is a heterojunction, and the n-first cladding layer contains In as a Group III element whereas the n-second cladding layer does not contain In. Further, the n-first cladding layer contains P as a Group V element whereas the n-second cladding layer contains As as a Group V element instead of P.
An altered layer may be formed at such a heterointerface and may absorb light generated in the LD, resulting in reduced luminous efficiency. If the electrical resistance of the altered layer is high, an increased threshold current of the LD may result. Further, if the altered layer has high distortion, the LD may degrade.